There’s a big difference between attaching an attic bench to a wall (which shouldn’t have been done without permission) an annexing roof space to expand your apartment. Your neighbour should not have paved common property without permission either but two wrongs don’t make it right.
If nothing else, there is an established formula by which you have to compensate the Owners Corp for the loss of common property. Basically it’s the increase in value of your home thanks to the extra room minus the cost of construction. All of the balance goes to the Owners Corp. Even with that formula, which was established in a High Court ruling, you can’t assume that the other owners will allow you to do it. I would be taking a much more conciliatory attitude to this and I’d let the BBQ bench slide for now. If you want your attic room you’re going to need the support of 75 percent of owners to pass the exclusive use by-law required.